Photovoltaic cells utilizing semiconductor crystals for converting solar radiation to electricity are well known in the art and include many different kinds of crystalline or non-crystalline materials for accomplishing this purpose. A crystal which is widely used at present is silicon which is known to be most sensitive to wavelengths in the range of 0.6-0.9 micrometers and where the theoretical efficiency of the silicon crystal in converting the energy of radiation of such wavelengths to electricity is on the order of 50%. Other crystals, for example aluminium gallium arsenide, have been used for converting the energy of solar radiation to electricity and which have a range of wavelength sensitivity in the order of 1.6 micrometers for most efficient operation. As the photovoltaic cells, used to date, because of their relatively narrow range of sensitivity, effectively convert only a small part of the energy of the solar spectrum to electricity it has been proposed to combine solar cells which are sensitive to different parts of the solar spectrum to increase the effective amount of the solar spectrum converted. For example, it has been proposed to combine in a solar converter a silicon cell with an aluminium gallium arsenide cell and to utilize a filter to split the solar spectrum into areas and ranges which come within the sensitive range of each cell.
In still a further attempt to utilize more of the solar spectrum for conversion purposes, it has been proposed to shift radiation of shorter wavelengths to longer wavelengths in order to come within the particular range of sensitivity for a particular cell. In this instance, which is known as thermophotovoltaic conversion, the solar rays are shifted to longer wavelengths by a tungsten radiator which is heated to approximately 2000.degree. C., with unused infrared energy passing through the silicon cell being recycled back to the radiator to help heat the same.
None of the attempts to increase the conversion of solar radiation to useful energy as far as I am aware however have been directed to utilizing the energy of the radiation of longer wavelengths to which the photovoltaic cell is insensitive where this energy is transferred to a heat absorber spaced from the cell. It is therefore an object of my invention to provide an apparatus and method whereby the energy of radiations of longer wavelengths to which a photovoltaic cell is insensitive may be converted to useful heat energy by a solar thermal absorber.